Following the great victory of Beowulf against Grendel and his successful wounding of him, there is, temporarily, a moment of peace and calm at Hereot that allows Hrothgar to celebrate and reward Beowulf for his valour. However, after speeches, entertainment and the presenting of rewards, it is clear that this moment of happiness is something that is only passing. Grendel's mother lurks in the darkness, determined to get her own back, and she enters Hereot. When the thanes awake and grab their weapons, she reacts in fear, but as she flees she takes with her one of Hrothgar's most favoured men:
To Hrothgar, this man was the most beloved
of the friends he trusted between the two seas.
She had done away with a great warrior,
ambushed him at rest.
Hrothgar's sorrow is therefore caused by the loss of a follower who was, as the description makes clear, of massive importance to Hrothgar as a friend. Note the use of the superlative phrase "most beloved" out of all of his friends. This clearly was a man who was greatly valued and esteemed by Hrothgar. This is also important in terms of the plot as it makes it clear that Hrothgar now has suffered evil at the hands of Grendel's mother, and therfore must send Beowulf again to gain revenge and bring peace.
Beowulf's victory over Grendel early in the epic establishes Beowulf as a great warrior, possibly the greatest warrior in the world. The Geats and the Danes both celebrate his victory--thinking that the danger is passed and Herot is once again safe.
But no one realizes that Grendel's mother lurks at the bottom of the lake. When Grendel makes it back to the cave and dies, his mother sets her sights on revenge and attacks Herot that night.
Interestingly, it appears that Grendel's mother had some inside knowledge about the Danes. The man she kills is Hrothgar's oldest friend and closest advisor--did she know who he was and specifically target him, wanting to hurt Hrothgar as she had been hurt? It certainly seems so, though it's hard to imagine how. We can only infer that Grendel and his mother were more than just evil monsters mindlessly killing men; they actually had information about the Danes and their lives.